Actually Variacs are transformers, and can supply lots of current.
If you can find an old filament winding transformer (About 6.3 V at
2A) its output is strictly limited by the number of maggots that
can run around in its core. The number will be considerably larger
than 2A, but strictly limited. Of course, such an overload will
destroy the transformer in a short time, but that period is long
compared to the desired testing interval. For less current, find a
smaller transformer.

Hi Rick
I've described this methode before. Most people seem to think
that a method of finding the short requires feeding current through
the short. My method uses a different idea.
Put your supply across the 12v line, from one end to the other,
in current limit. The line then has a small voltage drop across it.
Place a DVM lead on the ground. The location isn't important
since there is no current in the ground.
Use the 200mv scale. This works best with a 4 or 5 digit meter.
With the other lead, probe along the 12V line. When the meter
reads closest to zero, you are at the location of the short.
I have used this method many times when other methods
seem to fail.
The idea is to consider the 12 v line to be like a resistor with
a tap on it. The ground it the tap. With the second lead as
another tap, when both taps are at the same location along
the resistor, there is 0 volts.
Dwight

Buy a bag of very "light" iron powder (I saw that on sale on USA); take
a thin glass panel out from a window, cover it with water and a drop of
soap, mix the iron powder in the water, and carefully put the whole
panel upon your board. Put many amps across the board.
Watch the magnetic flux lines around the current path.
Clean up the whole mess...

:-)

(I just looked a video of a guy building magnetic field "readers" with
bottles, water, and iron poweder... fun pictures!)

Seriously, when I was interested in knowing where the short happened, I
used some dedicated instruments; it was very quick and effective. I'm
not at work now and I don't recall the model.

Basically, a gradient search should tell you where the short is located.

But if you just need the board, I would simply blow it up ad many said.

I seem to recall a magnetic liquid with some very unusual properties.
I don't recall the exact form, but in all of the images I saw it was
intact and 3D. It would illustrate some very odd fields, one caused
it to look like a spiky soccer ball. Anyone know what it was?

Ferrofluid --- basically a fine suspension of iron particles in oil,
I think. The "spiky soccer ball" is pretty characteristic of what it looks
like near a magnet. I think the spikes are not originally from the shape
of the field, but because the surface is somewhat unstable in a strong
enough field.

Hi
The main trick is to get enough drop along the trace. Use as much
current as
you can. For smaller traces, I've used 1 to 5 amps someplace.
This trick even works for power plane to power plane shorts but you
need
to do two different directions across the plane that you are driving
power
across. You form two lines of zero volts. Where they cross is the
location of
the short.
If the 12v line has branches, you may find the zero volt is down a
branch. Just
move the location of one of the current injection points to the end of
the branch
and search along the branch for the zero volt location.
Dwight

In general, IMHO, it's best to use a beefy current source limited to
250mV or so. Then you can put it pretty much on any two points on a
board without blowing things up (well, except very low resistance
fuses, very low value resistors, perhaps some very low resistance
coils). You won't even be outside the specs on most chips, where
+/-300mV is typically allowed.
Best regards,
Spehro Pefhany

--
"it's the network..." "The Journey is the reward"
snipped-for-privacy@interlog.com Info for manufacturers: http://www.trexon.com

In reviewing some of the other replies I noticed that no one had yet (at
the top level of the tree) suggested using a signal injector and a trace
probe. By injecting a current driven signal onto the power rail and
ground you can prod around with the trace probe and determine where the
short is by the signal level dropping nearest the location.
Non-destructive methods are usually preferred.

Using a variable frequency RF-generator you should be able to
determine when the tracks and the short is resonated e.g. at 1/2 or
1/4 wavelength. After determining the PCB material and hence the
velocity factor, you should be able to determine the distance from the
feedpoint.

One other method would be to inject a sub-nanosecond pulse edge and
determine, how long it takes for the reflection from the short to
return back to the generator.

A different approach would be to inject a significant current into the
+12 V line and try to detect the magnetic field on the +12 V trace,
until it disappears into the ground plane. This should work even if
the +12 V trace is within a multilayer PCB. Some Hall sensor might
useful, if DC current is used to create the magnetic field, but if an
AC (mains /audio frequency) current is running in the trace, some
pick-up coil should be able to pick up the stray magnetic field.

As a matter of fact, the last day job I had the company had just
bought a $50,000 TDR. I am sure fixing a $150 board is exactly the
job they had planned for it. Even with that, I'm not sure it could
show me the short. I don't know for sure if it is in the boards or on
top of it. The TDR won't tell me that.

It wouldn't easily identify the physical location of the short, though
it might help narrow it down. In my case, a $150 used current tracer
(HP 547A) and a $150 used signal generator (HP 8654A) worked great when
I last had to solve this kind of problem. It led me right to a solder
bridge under a closed frame DIP socket. $300 in used tools can help a
lot more than a new $50K tool, if the $50K tool doesn't happen to be the
right one for the job. On the other hand, it would certainly be nice
to have a high-performance TDR at hand when I need one. I've always
had to make do with the signal generator and an oscilliscope.

I'd like to replace the 8654A with an Agilent 33220A, but I can't
really justify the expense. I haven't quite figured out why test
equipment resellers are charging more a used 33120A than Agilent
charges for a brand new 33220A.

It most likely wouldn't work very well. A really wide trace of plane
smears out the location too much.

With most of us being in the Wild West I am surprised nobody came up
with the obvious: Take a Smith&Wesson and blast holes into it until the
current drops. That's where the short is. Or rather, was. Just kidding ;-)